In general, a strut-type suspension is mainly used in a front wheel of a four-wheeled vehicle, and is constructed such that a strut assembly incorporating a hydraulic shock absorber in a cylinder formed integrally with a main shaft is combined with a coil spring. Among such suspensions, there is a type of structure in which the axis of the coil spring is actively offset with respect to the axis of the strut, so as to allow the sliding of a piston rod of the shock absorber incorporated in the strut to be effected smoothly. There is another type of structure in which the coil spring is disposed by aligning the axis of the coil spring with the axis of the strut. In either structure, a thrust bearing is disposed between a mounting member of a vehicle body and an upper spring seat of the coil spring to allow the rotation to be effected smoothly when the strut assembly rotates together with the coil spring by the steering operation.
In this thrust bearing, a rolling bearing using balls or needles or a synthetic resin-made sliding bearing is used. However, the rolling bearing has a possibility that fatigue failure occurs in the balls or needles due to fine vibrations and vibration load or the like, and there is a problem in that it is difficult to maintain smooth steering operation. The thrust sliding bearing has problems in that since its friction torque is high as compared with the rolling bearing, an increase in the thrust load results in an increase in the friction torque, which renders the steering operation heavy, and that, depending on a combination of synthetic resins, the stick-slip phenomenon occurs, frequently producing frictional noise attributable to the stick-slip phenomenon.
In addition, a lubricant such as grease is applied to the sliding bearing, and the above-described frictional noise practically does not occur insofar as such a lubricant is present, as desired, on the sliding surface. However, there can be cases where the frictional noise begins to be produced due to the dissipation or the like of the lubricant over long periods of use.
It should be noted that the above-described problems do not necessarily occur only in the thrust sliding bearing incorporated in the strut-type suspension, but can similarly occur in general thrust sliding bearings.
The present invention has been devised in view of the above-described aspects, and its object is to provide a thrust sliding bearing in which even if the thrust load increases, the friction torque practically does not change to make it possible to form the sliding surface with a low friction torque and maintain such a low coefficient of friction over long periods of use, and which is free of the occurrence of frictional noise at the sliding surface, and, when incorporated in the strut-type suspension, makes it possible to secure smooth steering operation equivalent to that of the rolling bearing.